The present invention is directed to a spray chamber and a system and method of applying liquid to solid particles.
Spray chambers are known having one or more spray nozzles for directing a spray of liquid onto a solid material, such as a large solid surface, or even onto solid particles such as food pellets, coal and seeds. In many known spray chambers, liquid is sprayed or directed onto a free falling curtain or falling mass of particles, such as disclosed in U.S. Pat. Nos. 2,197,792; 2,862,511; 3,288,052; 4,491,608; 4,921,674; 5,287,801 and 5,993,903. It should be noted that the term xe2x80x9cspray chamberxe2x80x9d as used herein is meant to describe not only chambers where a conventional spraying device, such as a pressurized nozzle is used to dispense liquid, but also any chamber where liquid is applied whether by actual spraying or by other means such as slinging or dripping.
In some known spray chambers, the particulate materials are directed to impinge upon or slide down flat plates or conical surfaces that have constant slopes, either while the particles are being sprayed, or while being directed to a free fall spray area, such as disclosed in U.S. Pat. Nos. 2,197,792; 2,862,511; 3,288,052; 3,716,020; 4,491,608; 4,921,674; and 5,993,903, as well as in Canadian Patent 937,552 and Soviet Union Patent No. 1,411,019. In other spray chambers, the particles are sprayed while in a rotating drum or similar chamber, or mixing of the particles in such a rotating chamber is combined with the spraying, such as in U.S. Pat. Nos. 3,101,040; 3,288,052; 3,716,020; 3,841,262; 5,443,637; and 6,056,822, as well as in U.S. Defensive Publication No T927,005. Finally, in some known spray chambers, the particles are projected into a ballistic trajectory and are sprayed with liquid while in such a trajectory, such as in U.S. Pat. No. 2,685,537.
In these known spray chambers, there are many drawbacks, including a lack of precise control for dispensing a liquid onto particles, particulary in minute quantities of liquid relative to the particles, while assuring that the liquid is evenly distributed onto the particles. The lack of uniformity in the liquid application requires added mixing of the particles following or during spraying which, in turn, causes breakage of the particles. The resulting smaller particles typically contain a larger proportion of the applied liquid, and control over the process can be lost altogether when the same particles become lost due to their small size. Another disadvantage often experienced is that of equipment surfaces which tend to build up spray or overspray. At best this represents added difficulty in cleaning. At worst the overspray is an indication that some liquid intended for application is instead being left behind.
In some applications, minute quantities of liquid must be applied to the solid particles, in a precisely controlled manner, and in a manner where the liquid is not subjected to extreme conditions such as high temperature or high pressure. For example, in the process of producing feed pellets, such as chicken feed, minute quantities of liquid enzymes, vitamins and other additives need to be added to the feed pellets after the pellets have been formed by a process involving high pressure and possibly high temperature. Oftentimes the constituents of the liquid cannot be subjected to the high temperatures and/or pressures without losing their potency. For this reason, the liquids must be applied after the pellets are formed. It is important for the additives to be evenly distributed onto the pellets and that the pellets remain unbroken. Mixing the pellets by tumbling, etc., after application of the additives is not helpful and could be detrimental in that the pellets are highly absorptive, so liquid applied to the pellets will remain on the originally sprayed pellets and will not be distributed onto other pellets merely by mixing the pellets, and the mixing process subjects the pellets to impacts which could lead to breakage of the pellets.
The present invention provides a spray chamber and a system and method of spraying which overcomes the problems in the prior art and allows for a precise amount of liquid, in a relatively minute amount, to be applied to particles in a uniform manner and without requiring separate mixing, while preventing the buildup of overspray on any surfaces which would require separate cleaning.
An integrated system is provided of innovative components which allows for highly predictable results, ease of manufacture and installation as well as control and maintenance.
Particles, which may be pellets, such as animal feed pellets, which are awaiting downstream application of liquid are introduced into the system at a surge hopper. The particles leave the surge hopper and flow downwards into a hood or plenum, which is positioned over a moving surface such as a conveyor belt. A rectangular aperture in the hood permits the particles to flow onto the conveyor in a controlled manner. The aperture is open in the direction of belt travel. The dimensions of the aperture may be adjustable. The resulting ribbon of particles is conveyed away from the plenum and represents a well-shaped pattern of flow. A motor which drives the belt has its speed closely controlled. The shaped flow of particles has a substantially constant volume of flow as the particles enter a mass flow-measuring device.
Variations in the density of the flow of particles are compensated for by a determination of the true mass flow of particles by the mass flow-measuring device. Together with a source of constant volume flow, corrections to the belt speed made by real-time determinations of mass flow by the mass flow-measuring device produces a substantially constant mass of flow as the particles leave the mass flow-measuring device. The particles accelerate during flow through the mass flow-measuring device and begin to separate in space, one from another. The curtain of particles then leaves the mass flow-measuring device in a stream after flowing across a pan. The curtain of particles is then sent into the spray chamber at a certain angle, which is imposed upon the flowing curtain of particles by the pan.
The spray chamber includes an interior passageway, preferably formed of stainless steel, which has a rectangular cross-section that varies in dimension along the path of the particles. The path of ingress into the spray chamber is first against a surface arranged to tangentially intercept the flow of the particles leaving the mass flow-measuring device. The departure angle for the flowing curtain of particles from the pan is commensurate with the angle of ingress into the spray chamber. The spray chamber does not permit the flow of particles to depart from a path of tangential flow along interior surfaces, and the flow of particles is instead made to slide along curved interior surfaces. In a preferred embodiment, the passageway is designed to send the particles by virtue of its vector, first along one wall with a sliding action and subsequently along an opposite wall with the same sliding action. The walls are curved and the path of the particles resembles an xe2x80x9cSxe2x80x9d. Along the described path for the particles through the spray chamber, a liquid is applied to the particles in at least one point, and preferably at at least two separated points in such a manner that any overspraying which may occur is ultimately swept from the same surfaces by the continuing particle flow. The passageway is not sealed, but open at the ends to permit the particle flow. However, the passageway is largely closed to achieve the advantage of closely controlled application of liquid and limited dust and mist escape. Furthermore, the design allows the particles to take a longer sweeping path and prevents the particles from decelerating. Therefore, clogging is eliminated and the particles are not subjected to impacts which could lead to breakage.